The present invention relates to an inverter for transforming a direct-current (dc) voltage into an alternating-current (ac) voltage.
Conventionally, in an inverter constituted with two power switching elements connected in series to a main power source (dc voltage source), in order to transmit a signal from control instructing means such as a microcomputer to a gate or base driver circuit of each switching element, there has been commonly adopted a method in which insulating means such as a photocoupler or a pulse transformer is arranged therebetween. This is because of necessity of level shift to cope with a floating voltage of a reference potential of the signal to be send to the driver circuit driving the positive side switching element, the reference potential changing from 0 volt to a voltage value of the main power source. Moreover, the control instruction means is insulated from the driver circuit for a high reliability. In regard to the method of transmitting a signal to the driver circuit via such an insulating means, a driver circuit adopting a photocoupler has been shown in FIG. 9 (d) of page 76 of "Denshi Gijutsu (Electronic Technologies)", November, 1986. In this circuit, when a voltage of the driver source power is lower than a predetermined value, a low voltage is developed from a source power operating the coupler. Consequently, the coupler is not turned on and hence a switching element in an output stage is kept remained in an off state. With provision of the driver circuit for the switching element of each of the positive and negative sides, it is possible to prevent, at an initial setup time in which the switching elements are powered, the elements from simultaneously activated to be turned on, thereby avoiding occurrence of a short-circuited state.
There has been recently developed a power integrated circuit (IC) in which power switching elements and driver circuits thereof are formed in an integrated manner. In many cases, the output stage is constituted with inverters. Such an example of inverters formed in the IC structure has been described in pages 412 to 416 of the "MOTOR-CON, September, 1987 Proceedings". FIG. 8 is a block diagram showing the functional configuration of the inverter. In general, in an inverter formed in an integrated circuit structure, the insulating means above are not suitable for the circuit integration and are hence not included in the integrated circuit. Namely, the signal transmission to the driver circuit is accomplished according to a circuitry technique. On the other hand, in order to prevent the switching elements respectively of the positive and negative sides from being simultaneously turned on, an enable terminal is adopted in the example of FIG. 8. Namely, only when an input signal to the enable terminal is set to a high level, the switching elements respectively of the positive and negative sides can be turned on.